[WIP] Add Pytroll governance document

[djhoese]

Overview

This PR adds the initial governance document for the entire Pytroll organization. As discussed during last week's status meeting, I think it would be a good idea for us to define the decision making process. It should help explain how and why decisions are made in the Pytroll community and the software projects under the Pytroll organization.

As mentioned during the call, this is based on the scikit-image governance document (https://scikit-image.org/docs/dev/skips/1-governance.html), but removes the need for official "improvement proposal" documents.

I understand if this seems unnecessary; along the lines of "everything is going fine as is". However, I think this helps newcomers know how things work and helps us if we have disagreements in the future.

I'm marking this as a WIP since there are currently a lot of TODOs, but I wanted to get feedback before I spend a ton of time on the exact language.

Other thoughts

1. Add a label for steering council decisions (or maybe just "governance")?
2. Noticed their is a "Pytroll Manifest" document, I think this is supposed to be "Manifesto".
3. Steering council: Who's in it? It probably shouldn't be an even number, so 5 people? Panu, Adam, Martin, Me, and Stephan or Sauli?
4. Do we want to define some kind of periodic review of the steering council?
5. Other terminology?

[djhoese]

There is a new "Minimal Working Governance" project that seeks to define a starting point for governance structure: https://github.blog/2021-07-22-minimum-viable-governance-lightweight-community-structure-foss-projects/

I haven't read the actual document, only this blog post, but we could probably adopt this as a starting point.